Quantifying Agricultural Non-Point Source Total Nitrogen Loads, Time-Lag Associations, and Management Outcomes in the Liao River Mainstream, Northeast China

Agricultural non-point source (NPS) pollution is an important contributor to nitrogen enrichment in many northern Chinese watersheds. Using monthly water quality data collected at nine national monitoring sections along the main stem of the Liao River during 2021–2025, we estimated total nitrogen (TN) emissions from crop cultivation, livestock and poultry farming, and aquaculture using an emission coefficient method. We also used the maximum information coefficient (MIC) to examine nonlinear associations between monthly discharge and TN concentration, and described the spatial and temporal patterns of TN. TN concentrations were lower at the upstream inflow section than at the downstream estuarine section, a pattern consistent with additional inputs within Liaoning Province. The MIC analysis identified a lagged statistical association between terminal discharge and TN concentration; however, it does not establish a transport time or causal mechanism. TN concentrations declined in 2025 while pollution control measures were being implemented, although the observational record cannot attribute that decline to the measures. These results provide a basis for prioritizing monitoring and agricultural NPS control in the Liao River Basin.